INT Dome

General Description

The dome is a steel structure consisting of pre-fabricated elements which
are welded to form the hemispherical dome. The dome roller bogies are enclosed
on the outside by a removable skirt including a weather seal and inside
by another removable skirt which incorporates a nylon brush seal. The dome
rotates on a flat top rail which is mounted to give a truly circular and
horizontal rolling surface. The axes of the 46 rollers are arranged such
that they slope inwards to intersect at a point which coincides with the
centre of rotation. This ensures that each roller is guided on a circular
path. Three of the rollers are connected to drive systems to provide dome
rotation. These are equally spaced around the dome. The dome position is
encoded by a 10 bit absolute T + R encoder giving positional accuracy
of ~ +/- 2 inches of travel. The encoder is driven by an endless chain
which is a fixed to the dome circumference.

The top and bottom shutters are of the 'up and over' type. Both are
driven via motors and gearboxes combined with a drum which carries the
steel driving cables. The motors and cable drums ride with the shutters.
Limit switches are operated by both shutters to enable and disable the
main drive or micro drives of each shutter.

Dome Control

The dome can be rotated locally from the BRITTAIN dome/shutter control
cabinet (which rides with the dome) when the keyswitch is put to LOCAL
or in either engineering or computer mode with the keyswitch
set to REMOTE. Both engineering and computer modes communicate with the
dome electronics using TEM-L equipment. In engineering mode, push buttons
on the engineering desk control the dome direction; the speed being set
using thumb wheel switches.

In computer mode, TCS control for dome direction and speed is via CAMAC
2407
output driver module in Branch crate (Address: B4 C3 N8 A0/A1).
Bits 1 and 2 of channel A0 are sent to TEM-L when in turn operates
the relays and contactors to the dome motors to start movement in a CW
or CCW direction respectively. Bits 1 to 8 (2 BCD decades) of channel
A1
set the dome speed. These are applied to a DAC circuit mounted in another
dome wall box which connect up to the TASC unit which governs the speed.
Normally these bits are all set high and the dome moves at it maximum velocity.

Dome position is measured using a 10 bit T + R absolute encoder.
The encoder and a line driver box are located just below the balcony floor
at the east end of the walkway. A steel cover plate needs to be removed
to gain access. The signals from the line driver box pass to
a CAMAC PR2403 input register (Address: B4 C3 N10 A0)

Refer to the INT
CAMAC manual for more details and TEM-L manuals for more details.

Shutter operation

The shutters can be operated from either the BRITTAIN control cabinet or
from the DOME/SHUTTER control panel located about midway up in the engineering
rack in the control room. The switches (some are keyswitches) on the Brittain
cabinet must be put into the REMOTE position to allow shutter operation
from the control room. n.b. The shutters CANNOT be operated
by computer control.

Two drive systems are incorporated on the TOP and BOTTOM shutters.

Raise or lower the TOP or BOTTOM shutter using MICRO DRIVE (slow speed)

Raise or lower the TOP or BOTTOM shutter using MAIN DRIVE
(normal speed)

Initially, the top shutter has to be raisedin MICRO drive
to clear the OVER MAIN TRAVEL limit. Once the limit has cleared (indicated
by a lamp switching off on the dome/shutter control panel in the engineering
rack) the top shutter can be raised using the MAIN drive. Just before reaching
its fully open condition, the top shutter will operate a limit switch and
stop. MICRO drive then has to be used again to move the top shutter to
its fully open position thus enabling the telescope to observe at the zenith.

Closing the shutter follows a reverse procedure, but using the lower
MICRO
and MAIN push buttons. The same applies to the BOTTOM shutter. It must
be raised or lowered in MICRO drive before the MAIN drive can be operated.

The bottom shutter is unique in that it also serves as a windshield.
It consists of two sections. Once the first section raises past the dome
aperture, the lower section is picked up and and they continue to move
as a pair. In normal observing, the bottom shutter is seldom used. The
only time being that if the telescope is observing close to the horizon,
the top of the bottom shutter (when closed) could vignette the beam. In
this case, the bottom shutter would be raised to allow the telescope to
observe through an aperture.

n.b. Although it was originally planned to encode the dome shutters,
this was never implemented. The Baldwin absolute encoders were removed
from the shutter drives many years ago as were the TEM-L boards (in Dome
Box 3).

Dome Slip Rings

To enable continuous dome rotation, two slip rings are used. The pickup
boxes are located behind the steel covers at the top of the stairs that
go to the balcony.

Supplies 415V 3 phase power for the dome, shutter
and the dome crane.

Provides a contact pair for communications between
the dome electronics and the control room (TEM-L)

Dome Telemetry Equipment TEM-L

The telemetry equipment (TEM-L) provides the facility for transmitting
contact states and TTL levels between the dome and the control room / CLIP
centre. The system utilises 3 state signalling techniques to transfer information
serially to and from the dome TEM-L wall boxes using a twin cable.

There were originally three Dome TEM-L wall boxes, but only boxes 1
and 2 are used now. n.b. Dome Box 3 contained TEM-L boards for shutter
encoding, but this was never implemented. The dome wall boxes contain a
strip heater operated by a thermostat. This prevents condensation forming
during cold periods.

The control room (engineering) TEM-L crate is located at the bottom
of the engineering rack. The CLIP centre (computer) TEM-L rack is located
at the bottom of bay 6

n.b. It should also be noted that some of the receiver/transmitter/extender
boards I/O's are either TTL or OPTO isolated depending on the equipment
to which they are attached.

To improve noise immunity; especially through the slip rings, the communication
line uses a balanced pair working at approx +/- 50V relative to ground.
These lines are fused on the line driver board within the TEM-L rack.

Since the various TEM-L transmitters share a common communication line,
the equipment works in half duplex mode. To enable several transmitters
to share the same line, a time share (multiplexer) module listens to the
line and allows its transmitter module to send information only when the
line is quiet. Once the transmitter has transferred all its data to the
communication line, it is switched off for a period long enough to allow
all the other transmitters to send their data. To ensure that the TEM-L
receiver modules decode information from the appropriate transmitter, each
transmitter/receiver module is set up with its own unique address which
is sent as a 4 bit data block preceding the main (16/32 bits) of data.

A 16 contact input transmitter module accepts input data for transmission
at the Dome wall Box 1. The data is available as 16 TTL outputs which are
fed to indicator switches on the engineering control rack. Two 16 contact
input transmitter modules accepts input data from the engineering rack.
This data is available as 16 opto-coupled open collector outputs at the
Dome wall box 2.

A full technical description of TEM-L is beyond the scope of the document,
but detailed information is available in the documentation filing cabinets
in the electronics workshop and in INT Technical manuals:56
and 57

Dome TASC unit

The Dome rotation motors are connected to three drive rollers via a combination
helical/wormdrive gearbox to a TASC unit which provides an infinitely variable
speed or torque drive. In effect, there is no direct mechanical coupling
between the motors and the dome wheels.

A poled rotor which is fixed to an output shaft is surrounded by a tube.
This is driven by a constant speed AC motor. A stationary brushless coil
held on the stator is fixed to an outer casting. Magnetic flux is set up
when the field coil is energised generating eddy currents in the tube and
causing the rotor to revolve. A tachogenerator mounted to the output shaft
of one of the drives generates a voltage proportional to speed. Cooling
air is provided by a constant speed fan, driven by the motor.

Speed control is achieved by comparing a reference voltage (supplied
by the speed control potentiometer or via the TEM-L system) along with
the feedback signal from the tachogenerator. Differences between these
voltages generate more or less current to the excitation coil thereby maintaining
the set speed constant. Additional facilities are provided to vary the
acceleration and the torque. Refer to the TASC unit technical manuals for
further information.